This invention relates to a video phone for transmission of audio signals and freeze-frame video images over voice grade telephone lines.
Video teleconferencing systems typically require dedicated communication channels with a high bandwidth in order to transmit the large amount of data required to represent a complex video picture as in the case of text or graphics. Such devices require a communication channel having a bandwidth in the megahertz (MHz) range. See, for example, U.S. Pat. No. 3,982,063.
Video transmission systems intended to operate over voice grade telephone lines must contend with the fact that the usable telephone bandwidth is only approximately three kilohertz. Because of this bandwidth limitation and a corresponding limitation in the amount of data that can be sent, some of these systems only send a single video frame, rather than a moving picture. In addition, a long time is required for transmission, usually in the range of thirty to sixty seconds or more. See, for example, U.S. Pat. No. 3,842,199, which discloses a video (only) transmission system and U.S. Pat. No. 4,099,202 which discloses a method for multiplexing a slow scan TV signal with a voice signal.
When single frames of the video image are sent, the message header can become a significant portion of the transmitted information. Accordingly, the protocol used for the header is important. For an asynchronous system, a new header must be sent with each data package. Accordingly, the long headers used for synchronous systems noticeably lengthen the transmission time for asynchronous, single frame systems.
Freeze-frame video systems have other problems not associated with other telephone equipment. For instance, when a modem picks up a telephone line, there is typically a loud click noise due to the instantaneous application of a voltage across the line. This is not a problem for modems, since the phone is typically not at the user's ear when the modem is connected. However, for video phones, the telephone typically is at the user's ear when a picture is sent.
Freeze-frame video telephones have other problems due to the necessity for transmitting the pixel data in as short a time as possible, thus limiting the amount of error checking, etc. that can be used without lengthening the transmission time. For instance, DC voltage offsets can be picked up in the analog portion of the modem which are then transferred over to the digital portion when the pixels are decoded, introducing errors.
In another aspect of video phones common to modems, equalization is used at the receiver end to compensate for group delay of transmissions over the telephone line. This typically takes the form of four or five sections of all-pass filters with phase delay which is the inverse of the phase delay over the telephone line.